The evaluation of nonisothermal thermoanalytical kinetics is simplified without the description of heat transfers, such as thermal inertia, which is not negligible, as indicated by Vyazovkin

Abstract

In the field of thermoanalytical kinetics, which is widely used, basic questions arise regarding a possible description of the effect of heat transfer and subsequent thermal inertia. Current practice still uses a simplified interpretation of directly measured data for an inert sample at a constant temperature rise to describe the studied reaction, which leads to a simplified reaction conversion ratio, which is further used for various mathematical processing. The analysis of the introduced rectangular heat pulses and their indication by DTA clearly shows that the effect of thermal inertia is fundamental and not negligible. This is, after all, known from calorimetry, where the integral constant of inertia has historically been used to calibrate the descending temperature, but it cannot generally be applied to the whole temperature process due to its nonlinearity. In the case of parallel processing, for example by gradually increasing the heating rate, the effect of thermal inertia can be partially compensated for each other which have yet to be confirmed. However, for the kinetic analysis of a single peak, it is necessary to adapt to the effect of the thermal delay due to thermal inertia, which, similarly to the measured enthalpy, is tied to the sample size. Of course, it will be up to the researcher to find a solution that also includes the effect of heat transfer and dissipation, which should become the main principle of thermoanalytical methods of the future, as shown by current examples combining thermal processes with their specific kinetics.

Dynamic Character of Thermal Analysis Where Thermal Inertia Is a Real and Not Negligible Effect Influencing the Evaluation of Non-Isothermal Kinetics: A Review

“…It also follows in [14] that the result of the ICTA kinetic committee work [15] says nothing more that is logically obvious, i.e., that the kinetics are less affected by heat transfer the closer they are to the equipoise (isothermal) regime, e.g., introduced by decelerate heating. The remark in [13] also points to the negligibility of thermal inertia due to the decreasing sample size that was previously exposed on the samples of large and small cars in collision. This is because, for both larger and smaller sizes, the heat transfer is real even if corresponding to the size and involved heat capacity but affects the goal of measuring the determination of enthalpy changes.…”

“…Further, it has been said that as thermal inertia has not been applied in kinetics over the past 300 years, there is no reason to apply it now or in the future [11] as it will complicate calculations. Still others use philosophical resignation, saying that science is never accurate, and it only approximates the truth-as an excuse for knowingly using inaccurate and simplified thermodynamic models while the ways to correct them has been known for a long time [11][12][13]. If Newton, Fourier, Carnot and Tian were proponents of simplified continuity and motivated by today's job security, i.e., the need to maintain easier approaches, they would not bother to examine the impractical complexities and ineffectual intricacies of heat transfer, which form an inseparable basis for thermal analysis [14][15][16][17].…”

“…Although thermal analysis has thermal in its name, its practical application factually circumvents incorporation of thermal transfer relations. It manifests itself in the endless discussion of whether or not thermal inertia has an effect on the specific evaluation of thermo-analytical kinetics, which seems unceasing [11][12][13][14]. In [14], it was said that the effects of heat transfer in general are diminished by using smaller sample masses (m) and slower heating rates (β).…”

“…The (left) shown curves trace true-life measurements of temperature where the shaded area represents the deviation of programed temperature from that factually measured. This shows the magnitude of the discrepancy between the programed temperature (which is traditionally used as representative) and the actual value (which is, in fact, mostly ignored via simplification [13]).…”

“…In our sophisticated age of computers that bring the latest scientific knowledge, the lack of awareness of basic thermo-analytical theories [4,6]-particularly that of heat transfer physics [31,36]-as applied in nonisothermal kinetics is surprising, although in engineering thermodynamics heat transfer [35][36][37] is a matter of course. Obviously, the ignorance or reluctance concerning incorporation of the influence of Newton's law of cooling into the dynamics of thermal kinetics is almost scandalous [4,13,24,28]. The editors and journalists, however, mostly reiterate the view that after hundreds of years it is not clear whether someone would be interested in these complications at present [11], as they could greatly obscure the simplified concept of thermo-analytical kinetics.…”